Serum albumins provide valuable scaffolds to which bioactive molecules may be fused, either through genetic fusions or chemical fusions to improve the properties of the fused molecule(s) (Leger, R. et al. (2004). Bioorg Med Chem Lett 14(17): 4395-8; Thibaudeau, K., et al. (2005). Bioconjug Chem 16(4): 1000-8; Balan, V. et al. (2006). Antivir Ther 11(1): 35-45; EP 0 413 622; WO 90/13653; EP 1 681 304; WO 1997/024445; WO 01/79271). Albumins and albumin particles are also important for carrying and delivering drugs and prodrugs to their sites of action (Kratz (2008) Journal of Controlled Release, 132 (3), p.171-183). Fusion and particle technologies offer improved dosing regimes due to improved pharmacokinetic properties, such as half-life extension, and may improve bioavailability and protect the fused bioactive molecule from inactivation.
The biochemistry, genetics and medical applications of albumins are well characterized (“All about Albumin”, T. Peters Jr., Academic Press N.Y., and see for example, the world wide Web at http://www.albumin.org/). Human serum albumin (HSA, also referred to as HA) is the most abundant protein in human plasma at ˜60 g/L. The sequence of HSA is provided in SEQ ID No. 1. Natural variants of HSA occur and a list of know polymorphisms is given in Minchiotti et al. (2008). Hum Mutat 29(8): 1007-16., and see for example, the world wide web at http://www.uniprot.org/uniprot/P02768.
The production and purification of recombinant human albumins are well established (WO 95/23857; WO 00/44772; WO 2006/066595; EP 0 305 216; Sleep et al. 1990 Biotechnology (N Y). 1990 January; 8(1):42-6)) and include recombinant human albumin for pharmaceutical applications (Bosse et al. (2005). J Clin Pharmacol 45(1): 57-67). The three-dimensional structure of HSA has been elucidated by X-ray crystallography (Carter et al. (1989). Science 244(4909): 1195-8; Sugio et al. (1999). Protein Eng 12(6): 439-46). The HSA polypeptide chain has 35 cysteine residues, which form 17 disulphide bonds and one unpaired (free) cysteine at position 34 of the mature protein (Seq ID No. 1). Cysteine-34 has been used to for conjugation of molecules to albumin (Leger et al. (2004) Bioorg Med Chem Lett 14(17): 4395-8; Thibaudeau et al. (2005). Bioconjug Chem 16(4): 1000-8), and provides a precise, well defined site for conjugation. However, conjugation at cysteine-34 provides only one site for attachment of a single moiety thus there is no choice of conjugation site. Also, the provision of a single conjugation sites means that only one moiety can be conjugated to each albumin molecule. What is required is an albumin molecule which provides one or more alternative attachment sites.